1. Field of the Invention
The present invention is directed to electro-conductive deposition methods. More particularly, the invention is directed to a liquid toner development system, and pattern forming method and apparatus capable of electro-conductive deposition.
2. Description of Related Art
Electro-conductive patterning is generally within the field of “printed electronics”, in which electrical conductors, or passive or active electrical components are formed on a substrate. Printed electronics is normally an additive process, wherein material is deposited on a substrate to form a predefined pattern. Multiple layers may be deposited of the same material or different, and may require secondary processes such as drying or curing to create the final form of electrical conductors or electrical components.
Many industrial printing technologies have been used for electro-conductive patterning. For example, screen printing, flexography, gravure, offset lithography and inkjet printing have been used for electro-conductive patterning with each having certain advantages and disadvantages. In the case of screen printing, it requires patterns formed from screens which are placed on a substrate, flooded with ink, then excess ink removed. This is a rudimentary process which is inexpensive, but is not suitable for high production volumes. Since the screen pattern is fixed, is not capable of producing variable patterns within a single set of screens. Gravure printing relies on engraved printing plates wherein the engraved features are filled with ink, then transferred to a substrate. This method is capable of high-quality and high-speed patterning, but is not capable of producing variable patterns within a single set of plates. Inkjet printing relies on a voltage impulse to propel droplets onto a substrate, resulting in a finally formed pattern. Inkjet printing is capable of high-quality printing, but is normally limited to a print swath of less than one inch. And further, print defects inherent in the technology normally requires an overlapping print mode (“shingling”) for full quality printing. Thus, a swathing print technology such as inkjet is inherently slow unless many expensive printheads are ganged together to improve printing speeds. Therefore, inkjet is normally preferred for low-volume printing. Inkjet printers also require low viscosity fluids for proper jetting, resulting in inks with high levels of solvent is which must be removed after patterning. This results in a further decrease in net throughput. Inkjet printing is a digital technology capable of changing patterns from one substrate to the next, which is a favorable property.
Another printing technology not normally considered for electro-conductive patterning is electrophotographic (EP) printing. EP printing is a digital technology which relies on a multi-stage electrostatic transfer of printed particles, such as “toner”, to a substrate. It is a page-wide technology capable of high speed and is capable of changing patterns from one substrate to the next. The electrostatic transfer process normally requires toner capable of holding an electrostatic charge, such as is common with polymers. The polymers normally interfere with the ability to form electrical conductors or components.
Previous attempts at conductive toner are discussed below:
Japanese Patent Laid-Open No. 2004-184598 teaches a liquid developer comprising toner particles in a binder resin with several metal particles evenly dispersed in a dispersion agent. The metal particles are of silver and copper, having an average metal particle diameter from 0.2 to 1.2 μm. The toner particle diameter is from 2.5 to 8 μm, and the developer contains 50 to 90 weight percent of toner particles. This is claimed to make the line edges sharper and reduce the circuit pattern line width. But, the method of '598 requires a high sintering temperature (1,000° C., for example). This sintering temperature is incompatible with substrates used for printed circuit applications.
Japanese Patent Laid-Open No. 2004-048030 describes an electronic circuit manufacturing method and manufacturing apparatus using metal-containing charged toner particles with resin containing the metal particles. The metal is silver or copper, wherein the metal particle concentration rate is 10˜90 weight percent. It is pressure transferred to an intermediate transfer drum, then onto a substrate. The metal particles become the plating core and act like a catalyst for a plating response. It claims to reduce costs, increase productivity of multiproduct and small-quantity production and shorten the evaluation cycle for manufacturing. The method of '030 also relies on a plating process (electrolytic or non-electrolytic) to improve the electrical conductivity, is giving rise to environmental issues normally associated with traditional plating processes.
Japanese Patent Laid-Open No. 2005-181621 teaches a developer which uses toner particles of copper/silver alloys to obtain a golden color output. The composition of the alloy is adjusted to affect the golden color. It claims that the resolution of wiring circuit without causing poor conductivity and the golden-color output can be improved.
The objective of the '621 reference is not to achieve high electrical conductivity, but rather to obtain a specific golden color through the use of discrete alloy compositions. Additionally, a large amount of binder resin is used in order to improve the ability to control the charge, fixing, and dispersion, thereby reducing the opportunity for high conductivity. For Example, Example 1 in the document provides 70 parts of wax to 20 parts of metal particles and copolymers (such as lauryl methacrylate). Since '621 relies on an intermediate transfer process, a high level of wax and resins is required to achieve intended results.
These examples describe circuit formation methods using electro-photography as being superior to conventional lithography, screen printing, and inkjet methods in terms of simplicity, low cost and mass-productivity. However, achieving high conductivity with EP technology has yet to be achieved.
Other issues not taught in these references, but required for an EP technology capable of reliably forming electro-conductive circuits, is to provide high adhesion to a substrate and with little to no background fogging.
It is therefore an object of the present invention to provide a liquid electrophotographic toner, apparatus, and method capable of patterning electrical conductors and components having high conductivity, high substrate adhesion, and with little to no background fogging.